In known fuel injection systems, the fuel supply devices, i.e., fuel supply pumps, which are normally electric pumps, are so dimensioned as to be able to supply sufficient fuel for any engine condition, including a full-load (open throttle) operation. However, the average effective fuel quantity is substantially less than the maximum so that the unneeded fuel is returned to the fuel tank. As the fuel is subjected to circulation through the injection system, it undergoes substantial heating so that, after its return to the fuel tank, the fuel in that tank also becomes heated. This heating leads to evaporation of fuel components with low boiling point and the generation of bubbles in the subsequent passage through the fuel supply pump in which there exists a relative vacuum that enhances the formation of such bubbles. Furthermore, the heating of the fuel in the tank tends to increase the evaporation losses.
It is another distinct disadvantage of the known fuel supply systems that the electric fuel pump operates at full volume and power even when the engine is idling or is operating in the partial-load domain. Accordingly, the life expectancy of the fuel pump is reduced. Furthermore, the operating power, i.e., the electric current supplied to the electric fuel supply pump, is much higher than necessary for average engine operation as is the generation of pump noise.